1. Technical Field
The present disclosure relates to a method for separating, and more particularly to a method for separating metal nanoparticles from a colloidal metal solution.
2. Related Art
In recent years, metal nanoparticles, which becomes a new functional material with advantages of high specific area, high activity and low melting point, are applicable to electronic ceramic material, catalyst, photosensitive material, electrical contact material, hybrid material, alloy solder, low-temperature-thermal-conductive material and electrical conductive ink.
Take nanosilver (nano-Ag) for example. Preparation methods of nano-Ag particles comprise a physical method and a chemical method. The physical method is that thinning a bulk material by a mechanical force so that the particle diameter of the material is reduced to a required particle diameter. But the disadvantage of the physical method is that it is uneasy to obtain a particle with a diameter less than 100 nanometers (nm). The chemical method includes a wet chemical reduction process, a photochemical conversion process, an electrochemical process and an ultrasonic chemical process. The above-mentioned process may obtain the particles with the diameter less than 100 nm. Because of advantages of simple processing and easy operation, the wet chemical reduction process is generally adopted so far.
Although the wet chemical reduction process is generally used in laboratories and industries, colloidal nano-Ag solution formed in the wet chemical reduction process comprises a large number of impurities so that the colloidal nano-Ag solution cannot be the final product used in some applications directly. Moreover, in other applications, the nano-Ag particles are required to be a dry-powder type (the particles must be dried and small). Therefore, the nano-Ag particles must be further separated from the colloidal nano-Ag solution formed by the wet chemical reduction process so that the nano-Ag particles may be used in some applications. However, the nano-Ag particles are small with high surface energy and are electrically charged and a surfactant exists on the surface of the nano-Ag particles so that the nano-Ag particles may be stably dispersed in a liquid phase and the separation method may not be applied to separate the nano-Ag particles.
In a method for solid-liquid separation of the colloidal nano-Ag solution, only a high-speed centrifugal technology (the rotation speed of a centrifugal machine is greater than 8000 rotations per minute (rpm)) can be applied to separate the nano-Ag particles from the colloidal nano-Ag solution well. However, in the method for solid-liquid separation of the colloidal nano-Ag solution, equipment function and safety are highly required, the cost is high, and the productivity is low, so that the production cost may not be reduced and the powdered nano-Ag particles may not be formed in mass production. Further, the product of the nano-Ag particles taken out from a centrifugal bottle of the centrifugal machine is hard to be operated, and the nano-Ag particles are agglomerated seriously after the centrifugal solid-liquid separation and are hard to be separated from each other again. Therefore, how to separate the nano-Ag particles from the colloidal nano-Ag solution is a problem needs to be solved.
In Taiwan patent number TWI250969, a nano-Ag composition is disclosed. The nano-Ag composition may be continuously stabled with diameters less than 100 nm. A colloidal nano-Ag solution is dissolved in ammonia with silver oxide and mixes with a protective agent and hydrazine (NH2NH2.H2O). Nano-Ag particles obtained by a liquid-phase chemical reduction reaction on the colloidal nano-Ag solution may include impurities, such as unreacted composition or precursors. If the solid-liquid separation is not performed on the nano-Ag particles, impurity pollution occurs in the nano-Ag particles.
In Taiwan patent number TWI337892, a method for obtaining high-concentration colloidal nano-Ag solution is disclosed. The chemical materials comprise sodium dodecyl sulphate (SDS), polyvinyl pyrollidone (PVP), polyvinyl alcohol (PVA), sodium borohydride, hydrazine, formaldehyde, glucose, sodium citrate and sodium hydroxide. Although the above-mentioned method may obtain the colloidal nano-Ag solution with weight percent (wt. %) of 1.5 and an average diameters of 10 nm, the colloidal nano-Ag solution may contain excess unreacted reagent and high-concentration impurities (such as, sodium ions or acid ions) or toxic substance (such as, formaldehyde). When the excess unreacted reagent and high-concentration impurities may not be separated from the colloidal nano-Ag solution, applications of the colloidal nano-Ag solution may be limited.
Moreover, in U.S. Pat. No. 7,329,301, U.S. Pat. No. 7,270,694 and U.S. Pat. No. 7,591,872, these nano-Ag particles are obtained by performing wet chemical reduction processes. The above-mentioned methods may be applied with using some kinds of reducing agents, protective agents, solvents and chelating agents. With moderate reaction temperature and sufficient stir, high productivity of the nano-Ag particles is achieved. However, these patent descriptions does not discuss about the following separation between the nano-Ag particles and the colloidal nano-Ag solutions.
To sum up, the technological industry of forming nano-Ag, nanosilver/copper (nano-Ag/Cu) and nanocopper (nano-Cu) needs a low-cost, high-efficiency and improving method for separating metal nanoparticles from a colloidal metal solution.